The present invention relates to light emitting diode (LED) devices and, more particularly, to an LED device comprising a phosphor-converting substrate that converts a fraction of the primary light emitted by a light emitting structure of the LED into one or more other wavelengths of light that combine with unconverted primary light to produce white light.
With the development of efficient LEDs that emit blue or ultraviolet (UV) light, it has become feasible to produce LEDs that generate white light through phosphor conversion of a portion of the primary emission of the LED to longer wavelengths. Conversion of a portion of the primary emission of the LED to longer wavelengths is commonly referred to as down-conversion of the primary emission. An unconverted portion of the primary emission combines with the light of longer wavelength to produce white light. LEDs that produce white light are useful for signaling and illumination purposes.
Currently, state-of-the-art phosphor conversion of a portion of the primary emission of the LED is attained by placing a phosphor in an epoxy that is used to attach the LED to a reflector cup, which houses the LED within the LED lamp. The phosphor is in the form of a powder that is mixed into the epoxy prior to curing the epoxy. The uncured epoxy slurry containing the phosphor powder is then deposited onto the LED and is subsequently cured.
The phosphor particles within the cured epoxy generally are randomly oriented and interspersed throughout the epoxy. A portion of the primary light emitted by the LED passes through the epoxy without impinging on the phosphor particles, whereas a portion of the primary light emitted by the LED impinges on the phosphor particles, thereby causing the phosphor particles to emit yellow light. The combination of the primary blue light and the phosphor-emitted yellow light produces white light.
One disadvantage of utilizing the epoxy comprising the phosphor particles is that uniformity in the white light emitted by the LED is difficult, if not impossible, to obtain. This non-uniformity is caused by non-uniformity in the sizes of the phosphor particles mixed into the epoxy slurry. Currently, phosphor powders having uniform phosphor particle sizes generally are not available. When the phosphor powder is mixed into the epoxy slurry, the larger phosphor particles sink faster than the smaller phosphor particles. This non-uniformity in spatial distribution of the phosphor particles exists in the epoxy once it has been cured.
Therefore, obtaining a uniform distribution of the phosphor particles within the epoxy is very difficult, if not impossible, due to the non-uniformity of the sizes of the phosphor particles. This inability to control the sizes of the phosphor particles and their locations within the epoxy results in difficulties in producing LED lamps that emit white light in a uniform manner. Therefore, the quality of the white light produced by LED lamps may vary from one lamp to another, even for a given model manufactured by a particular manufacturer.
Attempts have been made to overcome the disadvantages of using epoxies mixed with phosphor powders by placing luminescent organic dye films on a lens that encases the LED. The dye is carefully positioned on the lens at a particular location so that it totally absorbs all of the primary light impinging thereon and converts the primary light to light of a longer wavelength. A fraction of the primary light emitted passes through the lens without impinging on the dye. The primary light that does not impinge on the dye then combines with the longer-wavelength light to produce white light. Since the dye totally absorbs the primary light impinging thereon, any variation in the fraction of the primary light that is summed with the longer-wavelength light is supposed to be eliminated.
However, this latter approach also has several disadvantages. The placement of the dye on the lens is subject to manufacturing uncertainties, which may result in variations in the white light produced. Also, dyes that are stable over long periods of time generally are not available. As a result, wide spread use of phosphor-converting dyes has not occurred.
Accordingly, a need exists for an LED device that performs phosphor conversion and that overcomes the aforementioned problems and disadvantages.
The present invention provides an LED device comprising a phosphor-converting substrate that converts primary light emitted by the LED, which is either blue or ultraviolet light, into one or more other wavelengths of light, which then combine with unconverted primary light to produce white light. The substrate is a single crystal phosphor having desired luminescent properties. The single crystal phosphor has the necessary lattice structure to promote single crystalline growth of the light-emitting structure of the LED device. Moreover, the thermo-mechanical properties of the substrate are such that the introduction of excessive strain or cracks in the epitaxial films of the LED device is prevented. The dopant concentration and thickness of the substrate are capable of being precisely controlled and tested before the LED device is fabricated, which allows the fraction of primary light that passes through the substrate without being converted to be predicted and controlled. Likewise, the fraction of primary light that is converted by the substrate into one or more other wavelengths is predictable and controllable. By precisely controlling these fractions, phosphor-converted LED devices can be achieved that produce uniform, high-quality white light.
In accordance with the preferred embodiment of the present invention, the substrate is comprised as a single crystal phosphor. The substrate preferably is a single crystal Cerium-doped Yttrium-Aluminum-Garnet (Y3Al5O12:Ce3+) compound, also denoted as YAG:Ce. The substrate luminesces yellow light in response to receiving primary light generated by the light emitting structure of the LED. A portion of the primary light generated by the light emitting structure of the LED passes through the substrate and remains unconverted. The unconverted primary light, which is blue light, then combines with the yellow light to produce white light.
In accordance with one embodiment of the present invention, the substrate converts only a portion of the primary blue light emitted by the light emitting structure of the LED into light yellow light. The yellow light then combines with the unconverted primary blue light to produce white light. In accordance with another embodiment of the present invention, the substrate is sandwiched between the light emitting structure and a reflective surface. The substrate absorbs all of the primary light propagating into the substrate and converts it into yellow light. The yellow light is reflected by the reflective surface toward the light emitting structure. The yellow light passes through the light emitting structure and combines with primary light emitted by the light emitting structure in a direction away from the substrate. The combined light produces white light.